The present invention relates to a switching circuit and, more particularly, to a matrix switching circuit adaptable to a cooking appliance.
FIG. 1 shows a conventional switching circuit comprising a first transistor Q1, a second transistor Q2, a first resistor R1, a switch SW, a second resistor R2, and a third resistor R3.
The first resistor R1, the switch SW, and the second resistor R2 are connected between the drain of the first transistor Q1 and the gate of the second transistor Q2. Between the switch SW and the second resistor R2, the third resistor R3 grounded is connected.
A voltage of about 10 V is applied to the sources of the first transistor Q1 and the second transistor Q2. When a voltage of VG is applied to the gates of the transistors Q1 and Q2, these transistors function as follows:
7.45 V.ltoreq.VG.ltoreq.10 V: conductive PA1 0 V.ltoreq.VG.ltoreq.5.8 V: non-conductive PA1 5.8 V&lt;VG&lt;7.45 V: unstable PA1 The first resistor R1.apprxeq.10 K ohm PA1 The second resistor R2.apprxeq.10 K ohm PA1 The third resistor R3.apprxeq.50 K ohm PA1 The contact resistance of the switch SW.apprxeq.5 K ohm
The values of the resistors are as follows:
When the switch SW is conductive, a connection point A between the first resistor R1 and the switch SW bears a voltage of about 8.5 V, and another connection point B between the switch SW and the second resistor R2 bears a voltage of about 7.7 V.
As the first transistor Q1 is conductive, a voltage higher than the threshold voltage of about 7.45 V(VT) of the second transistor Q2 by about 0.25 V is applied to the gate of the second transistor Q2. Then, the second transistor Q2 is conductive so that the drain of this transistor bears a voltage of about 10 V. The drain bears the zero voltage when the switch SW is non-conductive. Thus, the switch SW is operated.
Unfortunately, a voltage for making the second transistor Q2 conductive in response to the actuation of the switch SW is only higher than the threshold voltage VT of the second transistor Q2 by about 0.25 V.
Little allowance for the presence of a noise voltage is therfore provided. When a negative noise voltage of about 0.3 V is applied to the connection point B, the second transistor Q2 fails to become conductive in response to the actuation of the switch SW. Thus, a stable operation cannot be expected in the switching circuit of FIG. 1.